Light-emitting diodes, commonly referred to as "LED's" are semiconductor devices which convert electrical energy into emitted light.
As is known to those familiar with atomic and molecular structure of semiconductor materials and electronic devices, electromagnetic radiation, including visible light, is produced by electronic transitions that occur in atoms, molecules, and crystals. Furthermore, the color of light that can be produced from an LED is a function of the basic semiconductor material from which the LED is formed, and the manner in which the semiconductor material may be doped. As is further known to such persons, blue light represents one of the higher energy phenomena within the spectrum visible to the human eye. By way of comparison, higher energy transitions such as ultraviolet light are invisible to the human eye. Similarly, red light represents the lower energy end of the visible spectrum, and infrared, far infrared, and microwave radiation represent even lower energy transitions that are out of the range of the visible spectrum.
Only certain semiconductor materials have the capability to permit the type of electronic transitions that will produce blue light in the visible spectrum. One of these materials is silicon carbide (SIC) which can produce several different wavelengths of blue light. The characteristics of silicon carbide and the manner in which blue light can be produced using silicon carbide are thoroughly discussed in U.S. Pat. Nos. 4,918,497 and 5,027,168, both entitled "Blue Light-Emitting Diode Formed in Silicon Carbide." Both of these patents are assigned to the assignee of the present application. These patents are incorporated entirely herein by reference ("the '497 and '168 patents").
The increased availability of blue LEDs has, however, increased both the demand for the devices and for particular performance specifications. In particular, one important performance characteristic of an LED is the amount of light it can produce from a given amount of electricity, a relationship referred to as quantum efficiency. As the use of blue LEDs has increased, the demand for LEDs with higher quantum efficiencies has likewise increased.
There are, however, some particular aspects of silicon carbide which must be addressed when attempting to increase the quantum efficiency.
LEDs formed in more conventional materials such as gallium phosphide (GAP) provide a comparative example. Gallium phosphide's conductivity is generally sufficient for the entire device to light up as current passes across the p-n junction. Stated differently, current spreads relatively easily in gallium phosphide, thus spreading the generated light relatively easily as well. For lower conductivity materials such as p-type silicon carbide, however, the current does not spread as efficiently throughout the entire device, thus reducing the amount of emitted light that could otherwise be generated.
The conductivity of silicon carbide can be increased, of course, by increasing its dopant concentration. Increasing the doping level is a less desirable solution, however, because the increased doping lowers the transparency of the device, thus detracting from its overall performance.
Furthermore, producing a blue LED in silicon carbide requires various dopant and current injection considerations in a manner described thoroughly in the '497 and '168 patents.